Plant for transmitting electric power, including VSC-converter and DC/DC-converter

ABSTRACT

A plant for transmitting electric power comprises at least one VSC-converter (1). It comprises also at least one DC/DC-converter (33, 34) having two current valves connected in series and an inductance connected to a midpoint therebetween. The DC/DC-converter is through said inductance connected to a first of the poles of the direct voltage side of the converter and through a first output terminal connected to one of the current valves to the second of the poles so as to obtain an unbalanced step-up-transformation of the direct voltage between the two poles while obtaining a potential having a higher value on the second output terminal (27) of the DC/DC-converter connected to the second current valve than on said first output terminal (28).

This application is a 371 of PCT/SE98/00502 filed Mar. 20, 1998.

FIELD OF THE INVENTION

The present invention relates to a plant for transmitting electric powercomprising at least one VSC-converter adapted to convert direct voltageinto alternating voltage and conversely and has an alternating voltageside to which at least one phase of an alternating voltage network isconnected and a direct voltage side having two poles with a directvoltage thereacross.

The expression alternating voltage network is here to be given a verybroad meaning comprising not only the case of a conventional alternatingvoltage network, but all types of electric connections on which analternating voltage is present, such as for example output terminals ofan AC-generator. With respect to the direct voltage side this mayadvantageously be connected to direct voltage networks for transmittingpower, but it could have any other conceivable connections.

In order to illustrate but not in any way restrict the invention, theparticular application of such a plant for transmitting electric powerthrough a direct voltage network for High Voltage Direct Current (HVDC)may be mentioned. Such a plant has recently become known through thethesis "PWM and control of two and three level high power voltage sourceconverters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm,1995, in which publication a plant for transmitting electric powerthrough a direct voltage network for High Voltage Direct Current (HVDC)is described. Before the issuance of the thesis plants for transmittingelectric power through a direct voltage network for High Voltage DirectCurrent have been based upon the use of line-commutated CSC(CurrentSource Converter)-converters in stations for power transmission. Bydevelopment of IGBTs (Insulated Gate Bipolar Transistor=bipolartransistor having an insulated gate) for high voltage applications andthe suitability to connect them in series in valves of converters, sincethey may easily be turned on and off simultaneously, VSC(Voltage SourceConverter)-converters for forced commutation have now instead become analternative. This type of transmission of electric power between thedirect voltage network for High Voltage Direct Current beingvoltage-stiff therethrough and alternating voltage networks connectedthereto offers several important advantages with respect to the use ofline-commutated CSCs in HVDC. For example the consumption of active andreactive power may be controlled independently of each other and thereis no risk of commutation failures in the converter and thus no risk oftransmission of commutation failures between different HVDC-links, whichmay take place in line-commutation. Furthermore, it is possible to feeda weak alternating voltage network or a network without any generationof its own (a dead alternating voltage network). Further advantages arealso there.

In a plant of such design, but also in plants of the type of otherdesigns defined in the introduction, it is desired to manage withoutexpensive transformers and still have the possibility to obtain exactlythe voltage asked for on the direct voltage side. It is then desirableto obtain this while generating as low losses as possible on achievingthis voltage and at the same time to obtain as low demands as possibleupon the current handling capability of the components utilized forcreating the voltage from the alternating voltage delivered to thealternating voltage side.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a device of the typedefined in the introduction, through which the objects mentioned aboveare satisfied.

This is according to the invention obtained by providing the plant withat least one DC/DC-converter having two current valves connected inseries and an inductance connected to a midpoint therebetween, andconnecting the DC/DC-converter through the inductance to a first of thepoles and through a first output terminal thereof connecting to one ofthe current valves to the second of the poles so as to produce anunbalanced step-up-transformation of the direct voltage between the twopoles while obtaining a potential of a higher magnitude on the secondoutput terminal of the DC/DC-converter connected to the other currentvalve than on the first output terminal.

A voltage adaption of the direct voltage side may take place without anyexpensive transformers through such an arrangement of a DC/DC-converter.This converter will then function as a so-called "step-up-converter" forraising the voltage level, while it in the corresponding way may be usedfor lowering the voltage level, i.e. function as a so-called"step-down-converter", when power is transferred from the direct voltageside to the alternating voltage side. Another result of this arrangementis that the total direct voltage of an output terminal (the secondoutput terminal) of the direct voltage side is not applied to thecurrent valves of the VSC-converter, so that these do not have to holdsuch a high voltage and may accordingly be made of a lower number ofpower semiconductor devices connected in series and additional costs mayby this be saved. But through the fact that the entire voltage acrossthe two poles of the direct voltage side of the VSC-converter, i.e. thevoltage across the two current valves connected in series, are utilizedfor obtaining the step-up-transformation, instead of utilized a voltagebetween a pole and the ground therefor, a larger voltagestep-up-transformation at a given ratio of the DC/DC-converter isprimarily obtained. This means that a lower current handling capabilityis required for the components and the losses will be correspondinglylower at a given power transmittance. Thus, the demands upon currenthandling capability of components included in the VSC-converter may belowered through the unbalanced step-up-transformation, but it isprimarily possible to have a lower voltage of the converter at a givenoutput voltage of the DC/DC-converter, so that a lower number ofcomponents, especially switches of turn-off type connected in series, isrequired in the VSC-converter for handling the voltage there.

According to a preferred embodiment of the invention the plant comprisesat least two VSC-converters connected to one the DC/DC-converter each inthe way mentioned, the second poles of the two VSC-converters being eachconnected to an output terminal of a first additional theDC/DC-converter in common thereto, the inductance of the latterconverter being connected to ground for defining a zero potential level,and poles having opposite polarities than second poles of the twoVSC-converters and the two DC/DC-converters first mentioned are adaptedto deliver potentials having mutually opposite polarities on the secondoutput terminals thereof.

Not only the advantages mentioned above in unbalanced transformation areobtained in this way, but it becomes also possible to connect aplurality of VSC-converters to a bipolar output in common in this way,in which the second output terminal of the respective DC/DC-converterforms one of the poles of the output, so each of the twoDC/DC-converters produces a monopolar voltage of opposite signs. Thefirst additional DC/DC-converter is required for obtaining currentbalance. Thus, it will be possible to operate bipolarly while utilizingthe unbalanced step-up-transformation with a minimum of additionalcurrent valves (only the current valves included in the first additionalDC/DC-converter). Furthermore, it will be possible to interconnectconverters and by that alternating voltage networks located atcompletely different places in this way.

According to a second preferred embodiment of the invention, whichconstitutes a further development of the embodiment last mentioned, thetwo DC/DC-converters are adapted to deliver potentials of differentsigns but having the same magnitude on the second output terminals. Avoltage balance between the two direct voltage poles defined by thesesecond output terminals is thus obtained in spite of the utilization ofthe unbalanced step-up-transformation with the advantages associatedtherewith.

According to another preferred embodiment of the invention, whichconstitutes a further development of any of the two embodiments lastmentioned, at least one of the two VSC-converters is replaced by aplurality of VSC-converters, which are connected in parallel at thedirect voltage side thereof with direct voltage poles in commonconnected to the DC/DC-converters. Thus, this embodiment makes aconnection in parallel of a larger number of VSC-converters possibleshould this be desired before the voltage is step-up-transformed by theDC/DC-converter, which may be advantageously made, since the voltagewill be that high after the step-up-transformation carried out that thecurrent at the second output terminals in question of theDC/DC-converter will be comparatively low also when a high power istransmitted, which in this way may be transmitted by adding the partpowers from the different converters.

According to another preferred embodiment of the invention, whichconstitutes an advantageous further development of the embodiment firstmentioned, the two converters and the first additional DC/DC-converterform a unit. The plant comprises two the units, having the second outputterminal of one of the DC/DC-converters of each unit connected to asecond additional DC/DC-converter at an output terminal each thereof,the inductance of the second additional DC/DC-converter is connected toground for defining a zero potential level, the second output terminalsconnected to the additional second DC/DC-converter having differentpolarity signs, and the remaining second output terminal of theDC/DC-converter of the respective unit is connected to the inductance ofa separate additional third the DC/DC-converter, the first outputterminal of which is connected to the same output terminal of theadditional second DC/DC-converter as the unit in question through asecond output terminal is connected to so as, on the second outputterminal of the respective third additional DC/DC-converter, to delivera potential step-up-transformed further with respect to the input ofthis converter of an opposite sign for the respective unit. A parallelas well as a cascade connection of the DC/DC-converters is in this wayobtained, so that the currents on the direct voltage side may at all thetime be kept within certain limits. When the voltage is low the currentis distributed among many converters and when the voltage is high thecurrent will be low as a consequence of such a high voltage. Thus, veryhigh direct voltages may be obtained through this embodiment withrespect to direct voltages present on the direct voltage side of eachsingle converter. The plant may be extended further according to thisprinciple for additional step-up-transformation of the voltage.

According to another preferred embodiment of the invention theconverters are arranged in connection with different generators ofelectric power driven by wind power, in which a converter may in thisway be arranged at each generator and such generators located over alarger geographical region may be interconnected to a direct voltagenetwork in common, preferably for transmitting the power generatedthrough the direct voltage network to one or more stations remotelylocated for conversion thereof into alternating voltage to consumers ofelectric power.

According to another preferred embodiment of the invention the plantcomprises at least two DC/DC-converters, each having a pole each of thedirect voltage side of the converter connected to the inductance thereofand the opposite pole to the first output terminal thereof fordelivering a potential of a higher magnitude at the second outputterminals thereof than at the first output terminals thereof. A balancedhigh bipolar voltage may in this way be obtained through an unbalancedstep-up-transformation for each pole of the converter and the need ofcurrent handling capability of the components included in theDC/DC-converters and the losses be reduced, although in such a case itmay be more natural to connect the DC/DC-converters between one of thepoles and the ground instead of between the two poles.

According to another preferred embodiment of the invention, whichconstitutes a further development of the embodiment last mentioned, theplant comprises more than two DC/DC-converters, the DC/DC-convertersbeing connected in a cascade connection with the first output terminalof a third DC/DC-converter to the second output terminal of a first ofthe two DC/DC-converters and the inductance thereof to the second outputterminal of the second of the two DC/DC-converters first mentioned for acontinued unbalanced step-uptransformation of the direct voltage betweenthe second output terminals. It is in this way possible tostep-up-transform the voltage of the direct voltage side of theconverter to very high levels so as to enable low resistive losses inpower transmittance through a direct voltage network in this way throughthe use of comparatively few current valves with low demands uponcurrent handling capability of the components belonging thereto.

According to another preferred embodiment of the invention a directvoltage network for High Voltage Direct Current (HVDC) is directly orindirectly connected to the direct voltage side of the VSC-converter,which constitutes a particularly advantageous application of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description ofpreferred embodiments of the invention cited as examples.

In the drawings:

FIG. 1 is a very schematic diagram of a plant according to a firstpreferred embodiment of the invention,

FIG. 2 is a diagram of the plant shown in FIG. 1 simplified with respectto FIG. 1 by summarizing symbols,

FIG. 3 is a diagram corresponding to FIG. 2 of a plant according to asecond preferred embodiment of the invention,

FIG. 4 is a diagram corresponding to FIG. 2 of a plant according to athird preferred embodiment of the invention,

FIG. 5 is a diagram corresponding to FIG. 2 of a plant according to afourth preferred embodiment of the invention,

FIG. 6 is a diagram corresponding to FIG. 2 of a plant according to afifth preferred embodiment of the invention, and

FIG. 7 is a diagram corresponding to FIG. 2 of a plant according to asixth preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure of a plant for transmitting electric power according tothe invention is very schematically and in simplified way illustrated inFIG. 1, in which only the components having directly something to dowith the function according to the invention have been shown in thedrawing for facilitating the comprehension of the invention. The plantcomprises a VSC(Voltage Source Converter)-converter 1 in the form of aconventional so-called six-pulse-bridge adapted to convert directvoltage into alternating voltage and conversely. The VSC-convertercomprises three so-called phase legs 2, 3, 4, each consisting of twocurrent valves 5-10 connected in series, in which each current valveconsists of at least one breaker 11 of turn-on and turn-off type,preferably in the form of an IGBT, and a rectifying diode 12 connectedin anti-parallel therewith. A great number of IGBTs may be connected inseries in one single valve so as to turned on and turned offsimultaneously to function as one single breaker, wherethrough thevoltage across the valve is distributed among the different breakersconnected in series. Thus, the symbol 11 shown in FIG. 1 also comprisesa series connection of such breakers. The control of the breakers isperformed by pulse with modulation (PWM). One phase 13-15 of a threephase alternating voltage network is connected to the midpoints of therespective phase leg and it forms an alternating voltage side 16 of theconverter. On the direct voltage side 17 of the converter two capacitors20, 21 are connected in series between a first pole conductor 18 and asecond pole conductor 19 with a grounded midpoint so as to define thedirect voltage of the direct voltage side and to ensure that one of thepole conductors is on a potential +U_(d) and the other on a potential-U_(d). This constitutes conventional technique.

The plant comprises also a DC/DC-converter 22, which consists of twocurrent valves 23, 24 connected in series and being of a similar type asthe current valves of the six-pulse-bridge, an inductor connected to themidpoint between these current valves and a capacitor 26 connected inparallel with the current valves between the two output terminals 27, 28of the converter. The DC/DC-converter is through the inductance 25connected to a first of the poles 18 of the direct voltage side of theconverter and through a first output terminal 28 to the second of thesepoles 19. This means that the voltage potential of the first outputterminal 28 will be -U_(d), while it will be +U_(d) (2X-1) at the secondoutput terminal 27, so that accordingly an unbalancedstep-up-transformation of the direct voltage across the two poles takesplace. X is then the ratio of the DC/DC-converter, and this depends uponthe relationship between the conduction times of the two semiconductordevices 29, 30, which are alternatingly made conducting so as to applydifferent voltages across the inductance 25. If the ratio X is forexample 3, the output voltage on the second output terminal 27 will thenbe 5U_(d). This is to be compared with the case of connecting theDC/DC-converter directly between ground and a pole, wherein at the sameratio thereof the output voltage would be 3U_(d).

This means the advantages discussed above of a lower demand upon currenthandling capability of the components, since it is transformed up to ahigher voltage, at the same time as the losses become lower.

The plant comprises further a so-called fourth phase leg 31 with thesame design as the other phase legs, but connected between the poles 18and 19 and has the midpoint thereof connected to an inductance 32, whichin its turn is connected to ground. The fourth phase leg or "balancingconverter" is controlled through pulses applied on the semiconductordevices thereof with a frequency associated with the frequency by whichthe valves of the converter are controlled. The latter may typically beabout 2 kHz. A zero potential level may be defined through thisadditional phase leg, which is necessary for enabling a monopolaroperation obtainable through this plant, which is simplifiedlyillustrated in FIG. 2. A drawback of such a monopolar operation is,however, that electrodes are required, which in the HVDC-case are buriedin the ground and may cause problems. The possibility to monopolaroperation of a direct voltage network is therefore primarily utilizedwhen there is a failure on one of the cables of the two pole conductorsin a network for bipolar operation. The contents of the two dashedframes 1' and 22' in FIG. 1 is in FIG. 2 summarized through the symbols1 and 22. The corresponding symbols for VSC-converters andDC/DC-converters have been used in the other figures.

A plant according to a second preferred embodiment of the invention isillustrated in FIG. 3 and it is constructed according to the sameprinciple as the plant according to FIG. 2, but it is modified forobtaining a bipolar voltage. The additional phase leg 31 is hereillustrated by dashed lines, since it is not necessary, but when it isthere it is possible to operate through half the bridge towards groundupon cable failure. Thus, this embodiment has two DC/DC-converters 33,34, which have a pole each of the direct voltage side of the converter 1connected to the inductance thereof and the opposite pole to the firstoutput terminal 28 thereof, so as to deliver a potential on their secondoutput terminals 27 having a higher value than on the first outputterminals thereof, namely +U_(d) (2X-1) and -U_(d) (2X-1), respectively.

A plant according to a third preferred embodiment of the invention isillustrated in FIG. 4, said plant comprising two converters 35, 36connected to a DC/DC-converter 37, 38 each in the same way as the plantin FIG. 2. The second poles 39, 40 of the two converters are connectedto an output terminal 41, 42 of a first additional DC/DC-converter 43 incommon thereto, the inductance of which is connected to ground fordefining a zero potential level. The poles 39, 40 having oppositepolarities form the second poles of the two converters. Under thecondition that the two DC/DC-converters 37 and 38 have a ratio X and thepotential of the respective pole of the two converters is +U_(d) and-U_(d), respectively, the potential of the output terminals 44, 45,which may form pole conductors of a bipolar direct voltage network, willbe +U_(d) (2X-1) and -U_(d) (2X-1), respectively. It is then possible toplace the DC/DC-converter 43 so that the two converts 35 and 36 may beseparated geographically and for example be connected to the output of agenerator of electric power each driven by wind power, so that aplurality of such generators may in this way be connected to one and thesame direct voltage network. The DC/DC-converter 43 is used forobtaining current balance, and through the arrangement thereof,accordingly through only two additional valves, it is possible tooperate bipolarly.

A plant according to a fourth preferred embodiment of the invention isshown in FIG. 5 and constitutes a further development of the embodimentillustrated in FIG. 3, and this figure illustrates how an arbitrarilyhigh voltage may be obtained through cascade coupling ofDC/DC-converters 46-53. Thus, each step of connected DC/DC-convertersutilized the entire output voltage from the previous step for thevoltage step-up-transformation thereof, i.e. the entire bipolar voltageis used, so that the DC/DC-conversion gets maximally efficient withrespect to a current dimensioning of the converters and theDC/DC-converters and the losses are minimized. If we assume that theratio of the converters 46, 47 in step 1 is X1, of the converters 48, 49in step 2 X2, of the converters 50, 51 in step 3 X3 and so on, and inthe nth step illustrated through the converters 52 and 53 Xn, the outputvoltage will after the respective step be ±U_(d) (2X1-1), ±U_(d) (2X1-1)(2X2-1), ±U_(d) (2X1-1) (2X2-1) (2X3-1) and finally ±U_(d) [II (2Xn-1)].If we for example assume that n=3 and X1=X2=X3=4, the output voltage ofthe direct voltage side of this plant will be ±U_(d) ×343. This showsthat this DC/DC-conversion technique may without unrealistically manysteps perform impressing step-up- and step-down-transformations of thevoltage, respectively.

A plant according to a fifth preferred embodiment of the invention isillustrated in FIG. 6, the plant only differing from the plantillustrated in FIG. 4 by the fact that it is a plurality of converters54-57 that may be connected in parallel on the direct voltage sidethereof with direct voltage poles in common connected to the respectiveDC/DC-converter 37, 38, 43. The dashed lines indicate here thepossibility to connect an arbitrary number of converters in parallel inthis way. In this way may for example the series of generators ofelectric power in the form of an alternating voltage or current of anyconceivable type, such as such driven by wind power, be connected to thedirect current network in common, also if these are geographicallycomparatively widely separated. Thanks to the high voltage obtainable onthe direct voltage network the total current will nevertheless be thatlow that the direct voltage network may deliver the electric powergenerated by all these generators without too large losses.

Finally, a plant according to a sixth preferred embodiment of theinvention is illustrated in FIG. 7, in which the DC/DC-converters havebeen connected both in parallel and according to cascades. Two units 58,59 constructed according to the embodiment according to FIG. 4 are withthe second output terminal 60, 61 of one of the DC/DC-converters of eachunit connected to a second additional DC/DC-converter 62 at each outputterminal thereof, in which the inductance of the DC/DC-converter 62 isconnected to ground for defining a zero potential level. The units areso connected that the output terminals 60 and 61 connected to theDC/DC-converter 62 have opposite polarity signs. The remaining secondoutput terminal 63, 64 of the DC/DC-converter of the respective unit isconnected to the inductance of a separate additional thirdDC/DC-converter 65, 66, the first output terminal of which is connectedto the same output terminal of the additional second DC/DC-converter 62as the unit in question through one the second output terminal 60, 61 isconnected to so as to on the second output terminal 67, 68 deliver apotential further step-up-transformed with respect to the input of thisconverter and having opposite signs for the respective unit. The voltageof the output terminals 67, 68 will then be ±U_(d) (2X1-1) (2X2-1), inthe case that the ratio of the DC/DC-converters in the first stage is X1and the two 65, 66 in the second stage is X2. An advantage of this wayto make connections is that the direct currents are all the time keptwithin certain limits. When the direct voltage is low the current isdistributed among many converters, so that the current handlingcapability of the components included therein has not to be to high, andwhen the voltage is high the current will be low as a consequence of thehigh voltage.

The invention is of course not in any way restricted to the preferredembodiments described above, but many possibilities to modificationsthereof would be apparent for a man skilled in the art without departingfrom the basic idea of the invention as defined in the claims.

It is for example well possible to combine the plants according to thedifferent embodiments with each other, and the number ofstep-up-transformation steps or the number of VSC-converters included inthe plants may in principle be arbitrary.

The VSC-converter may be designed in another way than shown and forexample have a NPC-bridge.

What is claimed is:
 1. A plant for transmitting electric powercomprising:at least first and second VSC-converters, each adapted toconvert direct voltage into alternating voltage and convert alternatingvoltage into direct voltage, and each having an alternating voltage sideto which at least one phase of an alternating voltage network isconnected and a direct voltage side having first and second poles with adirect voltage thereacross; each of said VSC-converters being connectedto first and second DC/DC-converters, respectively, each of said firstand second DC/DC-converters having two current valves connected inseries and an inductance connected to a midpoint therebetween; saidfirst DC/DC-converter being connected through said inductance to saidfirst pole in said first VSC-converter and through a first outputterminal connected to one of the current valves and to said second polein said first VSC-converter; said second DC/DC-converter being connectedthrough said inductance to said first pole in said second VSC-converterand through a first output terminal connected to one of the currentvalves and to said second pole in said second VSC-converter; whereby anunbalanced step-up-transformation of the direct voltage between the twopoles in each of said first and second VSC-converters is produced whileobtaining a potential of a higher magnitude on a second output terminalof each of said first and second DC/DC-converters, respectively,connected to the other current valve than on said first output terminal;and whereinsaid second poles of said first and second VSC-converters areeach connected to an output terminal of a first additional saidDC/DC-converter in common thereto, the inductance of said firstadditional DC/DC converter being connected to ground for defining a zeropotential level, and wherein poles having opposite polarities form saidsecond poles of said first and second VSC-converters and said first andsecond DC/DC-converters are adapted to deliver potentials havingmutually opposite polarities on the second output terminals thereof. 2.A plant according to claim 1, wherein said first and secondDC/DC-converters are adapted to deliver potentials of different signsbut having the same magnitude on said second output terminals.
 3. Aplant according to claim 1, wherein at least one of said first andsecond VSC-converters is replaced by a plurality of VSC-converters whichare connected in parallel at the direct voltage side thereof with directvoltage poles in common connected to said DC/DC-converters.
 4. A plantaccording to claim 1, wherein said first and second VSC-converters, saidfirst and second DC/DC-converters and the first additionalDC/DC-converter form a unit, and wherein the plant comprises two saidunits having the second output terminal of one of the DC/DC-convertersof each unit connected to a second additional DC/DC-converter each at anoutput terminal thereof, and wherein the inductance of the secondadditional DC/DC-converter is connected to ground for defining a zeropotential level, the second output terminals connected to saidadditional second DC/DC-converter having different polarity signs, andwherein the remaining second output terminal of the DC/DC-converter ofthe respective unit is connected to the inductance of a separateadditional third said DC/DC-converter, the first output terminal ofwhich is connected to the same output terminal of the additional secondDC/DC-converter as the unit in question through a said second outputterminal is connected to so as, on said second output terminal of therespective third additional DC/DC-converter, to deliver a potentialstep-up-transformed further with respect to the input of this converterof an opposite sign for the respective unit.
 5. A plant according toclaim 1, wherein at least some of the converters are intended to beremotely arranged with respect to each other at sources for generatingelectric power delivered thereto through the alternating voltage networkconnected thereto, and wherein these VSC-converts are adapted totransfer this electric power to a direct voltage network in commonarranged on the direct voltage side thereof.
 6. A plant according toclaim 5, wherein the converters are arranged connected to differentgenerators of electric power driven by wind power.
 7. A plant accordingto claim 1, comprising means for defining a zero potential levelconnected to at least one of the poles of the VSC-converter.
 8. A plantaccording to claim 7, wherein said means is formed by a connection inseries of two current valves each consisting of a semiconductor elementof turn-off type and rectifier diode connected in anti-paralleltherewith and an inductance connected to the midpoint between the twovalves, the opposite end of which with respect to said midpoint beingconnected to ground.
 9. A plant according to claim 1, wherein said firstand second DC/DC-converters have each a pole of the direct voltage sideof the converter connected to the inductance thereof and the oppositepole to the first output terminal thereof for delivering a potential ofa higher magnitude at the second output terminals thereof than at thefirst output terminals.
 10. A plant according to claim 9, wherein saidfirst and second DC/DC-converters are adapted to deliver potentials ofthe same magnitude but with opposite signs at the second outputterminals thereof.
 11. A plant according to claim 9, comprising aplurality of DC/DC-converters connected in a cascade connection, withthe first output terminal of a first cascaded DC/DC-converter beingconnected to the a second output terminal of a first of two cascadedDC/DC-converters and the inductance of said first cascaded DC/DCconverter being connected to a second output terminal of the second ofthe two cascaded DC/DC converters for a continued unbalancedstep-up-step-transformation of the direct voltage between said secondoutput terminals.
 12. A plant according to claim 1, wherein saidVSC-converter is connected to an alternating voltage network havingthree phases and comprises therethrough a six-pulse-bridge.
 13. A plantaccording to claim 1, wherein a direct voltage network for high voltagedirect current (HVDC) is directly or indirectly connected to said directvoltage side of said first and second VSC-converters.